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Astrometric microlensing probes of the isolated neutron star population with Roman

This paper presents a realistic simulation of astrometric microlensing events using the upcoming Roman Space Telescope to characterize the isolated neutron star population, demonstrating that approximately 100 such events will be detectable and providing a framework for their identification based on natal kick distributions.

Original authors: Zofia Kaczmarek, Abby Halasi-Kun, Peter McGill, Scott E. Perkins, William A. Dawson

Published 2026-04-08

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CC BY 4.0

Original authors: Zofia Kaczmarek, Abby Halasi-Kun, Peter McGill, Scott E. Perkins, William A. Dawson

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Milky Way galaxy as a giant, bustling city at night. Most of the "citizens" are bright stars, like streetlights. But there are also invisible ghosts floating around: Neutron Stars. These are the ultra-dense, dead cores of exploded stars. They are so heavy that a teaspoon of their material would weigh a billion tons on Earth.

The problem? They are invisible. They don't shine, they don't pulse (unless they are very specific types), and they are incredibly hard to find. For decades, astronomers have been trying to count them and understand how they were born, but they've been like detectives trying to find a needle in a haystack without a magnet.

This paper is a simulation (a super-advanced computer prediction) of what will happen when the Roman Space Telescope launches in 2026. The authors are saying: "We can't see these ghosts yet, but we know exactly how to catch them using a trick called 'gravitational microlensing'."

Here is the breakdown of their discovery, using simple analogies:

1. The Trick: Gravitational Microlensing

Imagine you are looking at a distant streetlight (a background star). Suddenly, a ghost (a neutron star) floats between you and the light. Because the ghost is so heavy, its gravity bends the light from the streetlight, acting like a magnifying glass.

The Flash: The streetlight suddenly gets brighter for a few weeks.
The Shift: The streetlight also appears to wobble slightly in position.

This is microlensing. It's the only way to find these invisible ghosts without them emitting any light of their own.

2. The New Tool: Roman's "Super-Eyes"

Previous telescopes were like looking at the city through a foggy window. They could see the streetlight get brighter (photometry), but they couldn't see the wobble clearly.
The Roman Space Telescope is like putting on a pair of high-definition, super-steady glasses. It can see two things simultaneously:

The Brightness: How much the light gets amplified.
The Wobble: The tiny shift in the star's position (astrometry).

By measuring both, astronomers can calculate the mass of the invisible ghost perfectly. It's like weighing a ghost just by watching how it bends a beam of light.

3. The "Ghost" Problem: Natal Kicks

When a neutron star is born, it doesn't just sit there; it gets a massive "kick" from the explosion, shooting it out of its birthplace at hundreds of miles per second.

The Analogy: Imagine a cannonball fired from a cannon. Some are fired gently (low kick), others are fired with extreme force (high kick).
The Confusion: If a neutron star is kicked hard, it travels far away from the center of the galaxy. If it's kicked gently, it stays closer. The authors simulated four different "kick speeds" to see how this changes the number of ghosts Roman will find.

4. The Big Discovery: The "Spur"

This is the coolest part of the paper. The authors plotted all the simulated events on a graph.

The Main Crowd: Most objects (like normal stars and white dwarfs) form a big, dense cloud on the graph.
The "Spur": The Neutron Stars, especially the ones with fast kicks, form a distinct, isolated tail sticking out from the main cloud.

The Metaphor: Imagine a crowded dance floor. Most people are dancing in a tight circle. But the Neutron Stars are the ones running fast in a straight line away from the group, leaving a distinct "tail" of movement behind them.

Why it matters: If Roman sees a "ghost" in that specific "spur" tail, astronomers can say with high confidence: "That is almost certainly a Neutron Star!" without needing to do extra, difficult measurements.

5. The Predictions

The authors ran the numbers and found:

Roman will catch about 11,000 microlensing events in total.
About 100 of those will be Neutron Stars.
If they don't fill in the gaps between observation seasons (like taking a break every few days), they will miss about 38% of the events. It's like trying to count cars on a highway but closing your eyes every 10 minutes; you'll miss a lot.

6. Why Should We Care?

Finding these 100 Neutron Stars is like finding a lost library of secrets.

The Mass Gap: We don't know the exact weight limit between a heavy neutron star and a light black hole. Roman will help fill this "gap."
The Kick Mystery: By seeing how many are in the "spur," we can figure out how hard these stars are kicked when they are born. This helps us understand the physics of supernova explosions.
The Equation of State: It helps us understand how matter behaves under impossible pressure (like squeezing a mountain into a sugar cube).

Summary

This paper is a roadmap. It tells astronomers: "Get ready for 2026. When the Roman Telescope turns on, look for these specific 'wobbly, bright' stars in this specific corner of the graph. If you find them, you've found a Neutron Star. And if you find enough of them, you'll finally solve the mystery of how these cosmic ghosts are born and where they go."

It turns the search for the invisible into a game of "Where's Waldo," but with gravity instead of a red-and-white striped shirt.

1. Problem Statement

Isolated Neutron Stars (NS) are critical for understanding stellar evolution, supernova physics, and the equation of state (EoS) of dense matter. However, they are notoriously difficult to detect and characterize because:

Observational Bias: Most known NS are in binary systems or are pulsars, which do not represent the full population.
Detection Limits: Current microlensing surveys struggle to distinguish isolated NS from other compact objects (white dwarfs, black holes) or main-sequence stars based on photometry alone. Photometric parameters (Einstein timescale $t_E$ ) overlap significantly between classes.
Mass Measurement: Without astrometric data, mass measurements for dark lenses rely on uncertain Galactic model priors, leading to large degeneracies.
Natal Kicks: The velocity distribution of NS at birth (natal kicks) is highly uncertain (100–500 km/s), yet it is crucial for modeling supernovae and gravitational wave sources.

The paper addresses how the upcoming Roman Space Telescope (launching 2026), specifically its Galactic Bulge Time Domain Survey (GBTDS), can overcome these limitations by combining high-precision photometry and astrometry to detect and classify isolated NS.

2. Methodology

The authors conducted detailed simulations using the PopSyCLE (Population Synthesis for Compact-object Lensing Events) software, tailored to the Roman GBTDS survey design.

Galactic Modeling:
- Used the Besançon model (via Galaxia) for stellar populations and SPISEA for stellar remnants.
- Employed the SukhboldN20 Initial-Final Mass Relation (IFMR).
- Simulated four distinct NS populations with Maxwellian natal kick distributions having mean velocities ( $\bar{v}$ ) of 150, 250, 350, and 450 km/s.
- Black holes were simulated with a fixed mean kick of 100 km/s.
Survey Simulation:
- Modeled the Roman GBTDS footprint (approx. 1.7 deg²) with a 12-minute cadence over six 70.5-day seasons.
- Included gap-filling low-cadence observations (every 3 days) between seasons.
Detectability Criteria:
- Applied cuts for photometric signal ( $\Delta m > \sigma_{phot}/\sqrt{N}$ ) and astrometric signal ( $\Delta_{ast} > \sigma_{ast}/\sqrt{N}$ ).
- Required the event peak ( $t_0$ ) to occur within the survey window.
- Converted magnitudes to Roman's F146 filter and applied a limit of $F146 < 23$ .
Density Correction (Crucial Step):
- Recognized that PopSyCLE initially places remnants at fixed positions, ignoring the effect of natal kicks on their spatial distribution over time.
- Used StellarMortis (Sweeney 2025) to simulate the dynamical evolution of $10^5$ NS in the Milky Way potential.
- Applied re-weighting factors to the simulated lens events based on the ratio of post-kick to pre-kick NS densities in specific Galactocentric bins ( $r, |z|$ ). This corrected the yields to reflect realistic, older NS populations that have migrated out of the Galactic bulge.
Classification:
- Developed a Bayesian classifier to determine the probability of a lens belonging to a specific class (Star, WD, NS, BH) based on the observable parameter space $\log_{10} t_E$ vs. $\log_{10} \theta_E$ .

3. Key Contributions

First Realistic Roman NS Yield Estimates: Provided the first comprehensive prediction of isolated NS detection rates for the Roman mission, accounting for complex natal kick dynamics and density corrections.
Identification of the "Spur" Feature: Discovered a distinct, characteristic feature in the $\log_{10} t_E - \log_{10} \theta_E$ parameter space that is unique to NS lenses, particularly those with high natal kicks.
Mesolens Concept: Identified that NS in the "spur" are "mesolenses"—nearby, high-velocity objects that sweep large areas per unit time, making them high-probability lenses despite potentially low overall population density.
Public Data Release: Made all simulated microlensing event datasets (pre- and post-detectability cuts) publicly available via the popclass software package to aid future Roman data analysis.

4. Key Results

Event Yields:
- Roman is expected to detect approximately 11,000 microlensing events with both photometric and astrometric signals.
- Of these, ~100 events are expected to be isolated NS lenses (specifically 75–181 depending on the assumed mean kick velocity, after density correction).
- Without gap-filling observations, the NS yield drops by 38% (retaining only 55–72% of events depending on kick velocity).
The "Spur" Feature:
- In the $\log_{10} t_E - \log_{10} \theta_E$ space, NS lenses with high kicks ( $\bar{v} = 450$ km/s) form a diagonal "spur" offset from the main distribution of stars and white dwarfs.
- This feature corresponds to low $t_E$ (short duration) and high $\theta_E$ (large angular Einstein radius).
- NS in this region are characterized by being nearby (low distance) and having high proper motions.
Classification Performance:
- Using the $\log_{10} t_E - \log_{10} \theta_E$ space, the authors constructed samples with high purity (near 100% NS) by selecting events in the "spur" region, though this comes at the cost of lower recall (missing some NS).
- Photometry-only ( $\log_{10} t_E - \log_{10} \pi_E$ ) cannot distinguish NS from other classes effectively.
Dependence on Kick Velocity:
- Higher kick velocities create a more prominent "spur" but also reduce the overall NS density in the inner Galaxy (where the survey points), creating a counteracting effect on total yields.
- The distance distribution of NS lenses becomes flatter and more extended with higher kicks.

5. Significance and Implications

Direct Mass Measurements: Unlike previous surveys, Roman will provide direct mass measurements for ~100 isolated NS via the measurement of $\theta_E$ and $t_E$ , breaking the mass-distance degeneracy. This will provide unprecedented constraints on the NS Equation of State and the existence of a "mass gap" between NS and black holes.
Natal Kick Constraints: The shape and strength of the "spur" feature are sensitive to the natal kick distribution. Observing this feature will allow astronomers to constrain the mean kick velocity and distribution of NS, testing supernova explosion models.
Survey Strategy: The study highlights the critical importance of gap-filling observations (low-cadence monitoring between seasons) to maximize the detection of short-timescale events caused by high-velocity NS.
Future Follow-up: The authors recommend prioritizing "spur" candidates for immediate follow-up (e.g., spectroscopy) to confirm their nature, as these events offer the highest probability of being isolated NS with measurable masses.

In conclusion, this paper establishes that the Roman Space Telescope will revolutionize the study of isolated neutron stars, transforming them from theoretical curiosities into a statistically significant, measurable population, provided that the survey strategy includes specific cadence requirements and that the unique "spur" signature in astrometric data is utilized for classification.